Update Clojure maven for latest on CI server

Update maven build infrastructure to stop using oss-parent and use updated Maven 3 and sonatype plugins (similar to other changes made recently in all contrib projects).

• Removed oss-parent parent pom. This has been deprecated for years and is no longer recommended for use.

• Add snapshot repo (was previously pulled in via oss-parent)

• maven-compiler-plugin - update to latest version

• maven-release-plugin - update to latest version

• add nexus-staging-maven-plugin - current recommended plugin for releases to maven central, replaces most of the maven-release-plugin's work (old version of this previously in oss-parent)

• add maven-gpg-plugin for signing (previously in oss-parent)

• remove old release profile which was activated by oss-parent pom

Patch: mvn3.patch

It's difficult to test this completely outside the context of actually building and deploying snapshots and releases but the changes are very similar to those made for all contrib projects recently.